In MOS transistor circuitry, particularly in CMOS technology, it is commonly necessary to couple the output signals from sense amplifiers to an output pin of the integrated circuit. Sense amplifiers are used, for example, to sense binary output signals from random access memories (RAMs), or product terms from the term lines of programmable logic devices (PLDs). Such circuitry often uses a tri-state output which not only can be high or low, providing a binary 1 or a 0, but can also be placed in the "off" state where the output is "floating". These circuits often employ output enable circuitry which is capable of totally disabling (turning off) the tri-state output.
In the past, attention has been paid to reduction of turn-on noise in such tri-state output circuits, such noise resulting from turning the output from "off" to "on". For example, U.S. Pat. No. 4,723,108, assigned to the same assignee as the subject invention, discloses the use of a reference input voltage supply to drive a pull-down transistor which pulls the output pad down to ground at appropriate times. As shown in FIG. 1, labeled "Prior Art", this reference input voltage at node 6 is used to reduce noise on the output signal pad at node 1 when transistor M19 turns on. The reference input voltage at node 6 of this prior art circuit, connected to the gate of transistor M15, produces a current which controls the rising edge of the voltage at node 2 connected to the gate of transistor M19, which in turn drives the output of transistor M19 down to ground at appropriate times, such as, for example, when the output signal should be "0" in case of a positive logic signal. This current into its gate controls the switching time of transistor M19 by controlling the rate of change, di/dt, of its source-drain current.
It is conventional wisdom that the principal noise problem observed in tri-state circuits of this type occurs during changes in the tri-state output device from no current to high current (when the output transistor is switched from off to on). Accordingly, no one has considered noise which may occur when the output transistor is switched from high current to low current (on to off). However, certain failures began to occur in circuits of this type resulting in spurious output data, and no one knew what the source of the problem was. Unexpectedly, it was found, again referring to the circuit of FIG. 1 of the prior art, that the voltage on the source of transistor M19 connected to V.sub.ss tended to fall relative to the ground voltage on the external PC board as a result of noise in the circuit. Surprisingly, this decrease in voltage occurred when node 2 was being switched from high to low (on to off), and often resulted in a failure condition of the circuit which prevented the correct detection of a data signal input. This failure mode was not understood.
An unexpected solution to this problem was found. It was discovered that one could use two reference voltage sources, one which is the mirror of the other, one of which is coupled to the gate of a transistor coupled to the positive rail and the other coupled to the gate of a second transistor coupled to the negative rail. Using these two reference voltages, it was discovered that it is possible not only to reduce output noise when the output transistor turns on, but also to reduce output noise when the transistor turns off. In addition, the circuit of the invention, contrary to that of the prior art shown in FIG. 1, is capable not only of controlling the switching of pull-down transistor M19, but also the switching of pull-up transistor M18, the noise on both of these switching transistors being controlled, as in the prior art, when they switch from off to on, and also when they switch from on to off.